The human body has electrical characteristics which can be measured for characterizing organ function and for the application of different therapies. For instance, the heart is a complex network of nerve and muscle tissue which operates in synchrony to pump blood throughout the body. Cardiac function may be monitored by sensing the electrical signals naturally conducted at certain places in the heart.
Sometimes it is convenient to apply signals to the body to determine the function of organs of the body. One way to apply signals is to use an implanted series of electrodes which apply a known current and measure the resulting voltage. The relationship between applied current and measured voltage is known as impedance. Thus, impedance is measured by injecting a known current using electrodes and monitoring the electrical voltage required to pass the known current between electrodes. The higher the magnitude of impedance, the higher the magnitude of voltage measured across the load for a known current magnitude.
If the electrodes are placed such that the impedance is measured across a right ventricular portion of the heart, then the impedance measured is a function of the stroke of the right ventricle. The stroke volume of the right ventricle provides a measure of the blood volume pumped by the heart into the lungs in one stroke.
The change in impedance is due to the conductive nature of blood and its changing volume in the left ventricle between contractions. The measured impedance will vary depending on the placement of the electrodes. For example, as shown in FIG. 1A and FIG. 1B, if a current is conducted between the housing of an implantable device 12 and a tip electrode 13 on the end of a catheter 14 with the tip electrode 13 positioned in the apex of the right ventricle 15, then the impedance observed between two electrodes, 16 and 17, located within the right ventricle (and before the tip electrode 13) will measure an increased impedance for a contracted ventricle (systole--FIG. 1B) as opposed to when the ventricle is not contracted (diastole--FIG. 1A). This is because in diastole, the ventricle is holding more blood and has more conductive volume to transfer current. In systole, the ventricle is contracted and has less blood, leaving less volume for conduction.
A system for indicating the stroke volume of the heart by tracking the impedance changes of the ventricle through contractions is shown in block diagram form in FIG. 2 through 4 of my commonly assigned copending patent application entitled System for Processing Bursted Amplitude Modulated Signals Using an Impedance Sensor, Ser. No. 09/297,004 filed Feb. 8, 1999. In that application there is shown a low power processing system for processing bursted amplitude modulated signals by performing impedance-related measurements across a load. The system operates by injecting current pulses of constant amplitude across the load using at least a first electrode and a second electrode, the current pulses including bursts of a plurality of pulses at a pulse frequency at which the current pulses are repeated, the bursts transmitted at a burst frequency. It includes detecting voltages across at least a third electrode and a fourth electrode; high pass filtering the voltages to produce filtered voltages; amplifying the filtered voltages to produce amplified voltage signals. It also includes bandpass filtering the amplified voltage signals with a bandpass filter with a center frequency equal to approximately the pulse frequency to generate first filtered signals; rectifying the first filtered signals to produce rectified signals; integrating the rectified signals to produce integrated signals; sampling-and-holding the integrated signals after each burst to capture an integrated pulse value for each burst, creating a plurality of discrete integrated pulse values. It also includes further bandpass filtering of the plurality of discrete integrated pulse values using a filter including an upper cutoff frequency less than the burst frequency to produce the output related to the time-varying impedance of the load.
In the system shown in my prior application referred to above, the realization approach used in the first bandpass filter 42 is continuous time filtering. Because the continuous time filter technique does not use a sampling clock, it is able to process a high frequency signal.
A potential disadvantage for utilization of this type of filter circuit may be the need for tuning circuitry. Tuning may be required because the filter coefficients are determined as a product of two dissimilar elements such as capacitors and resistors (or transconductors). Although the variation of values of capacitors in integrated circuits is small, in the order of .+-.5% , the variation in resistors may be .+-.50%. Another characteristic of continuous time filters is the presence of flicker noise and poor linearity. All of these characteristics are addressed by the present invention which provides an improved realization of a transconductance gain cell that is particularly adapted for use in a bandpass filter realized from a continuous time filter.
Thus there is a need in the art for a self-adjustable continuous time band pass filter with a transconductance cell having bipolar transistors and a self adjusting bias circuit to stabilize the overall transconductance of the transconductance cell.